Conventionally, to measure the amplitude of a wavelength-multiplexed optical signal, the optical signal to be measured is normally split into optical signals of the respective wavelengths by a wavelength separation device, and the amplitude is then measured based on the signal waveforms of the respective wavelengths by a measurement device. The wavelength separation device may be a multilayer film optical filter or a diffraction grating disclosed in Patent Document 1. However, in a case where wavelength intervals are set in a dense manner as in dense wavelength-multiplexed optical communications, to accurately perform a wavelength separation, the physical locations and temperatures of those elements need to be controlled with extremely high precision.